Laser transmission welding is an established joining technology in many industries, including automotive engineering, electric and electronic industries. Laser welding is used to produce a wide range of products such as polymer system components, distance sensors, electronic vehicle security key systems, audio devices or blood pressure gauges.
Fiber-glass reinforced semi-crystalline polymers exhibiting high transparency to NIR laser light with their high heat performance, excellent chemical resistance and mechanical properties are potential candidates for the aforementioned applications. As a general tendency, the higher the degree of crystallinity, the higher the mechanical strength, heat resistance and chemical resistance. However, semi-crystalline polymers tend to scatter light due to coexistence of the amorphous and crystalline phases. Furthermore, incorporation of reinforcing agents such as glass fibers into the polymer matrix significantly decreases the level of NIR transmission due to increased light scattering. Consequently, transmission laser welding with fiber-glass reinforced semi-crystalline materials is hard to achieve and existing material solutions are limited to slow scan speeds, which is not very attractive, as it prolongs part assembly cycle time. Besides, the NIR transmission of a PBT based material is highly sensitive to processing conditions and variations in the composition. Therefore, robust manufacturing of laser transmission weldable glass filled blend compositions containing crystalline PBT is still a challenge.
The principle of transmission welding depends on the different optical properties of the polymers to be welded. A transparent and an absorbing part are positioned and clamped in an overlap configuration with the transparent part facing the laser radiation. A laser beam penetrates through the laser-transparent part with minimal loss of energy and is absorbed by the laser-absorbing part, which subsequently heats up and melts. Heat transfer through conduction leads to melting of both parts, thus generating a weld at the interfacial zone. For laser transmission welding, it is very important to achieve a sufficient and consistent heating of the polymers at the joint interface during pre-melt and fusion phase. The magnitude of the deposited energy at the interface can be controlled to some extent by adjusting laser welding conditions (laser beam power, welding speed, laser beam/spot diameter, clamp pressure) but depends largely on the polymer properties.
Poly(butylene terephthalate) (PBT) provides processing latitude in injection molding and in extrusion techniques owing to its high flow when molten and rapid crystallization upon cooling. Rapid crystallization offers processing advantage, particularly quick demoldability and short cycle times. Additionally, PBT offers excellent mechanical and electrical properties with robust chemical resistance, which is among key requirements for various industrial sectors including automotive, electric and electronic industries. Improved thermal resistance gained by reinforcing PBT resins with glass fibers make them outstanding candidates for applications in which the products are subjected to short-term heat exposure.
A potential problem with welding materials based on semi-crystalline polymers is that such resins scatter light due to coexistence of the amorphous and crystalline phases. Back scattering results in a reduction in the total amount of transmitted energy, whereas diffuse scattering leads to broadening of the laser beam. Consequently, the level of the laser energy arriving at the interface is diminished, thus decreasing the adhesion between the parts to be welded. In order to arrive at an acceptable weld strength in these low NIR transmitted materials welding speeds have to be reduced causing on significant increase in assembly cycle time. Incorporation of reinforcing agents such as glass fibers into the PBT resin significantly decreases the level of NIR transmission due to increased light scattering. Another potential problem with internal scattering of the laser light, especially in thick-welded parts, is that a significant rise in temperature can occur which may lead to weld instabilities. Hence, materials with high thermal resistance are highly preferred as transparent (upper) layer in laser welding applications. It is therefore desirable to combine thermal resistance and excellent mechanical properties of glass filled semi-crystalline materials with high laser transparency. To this end, there has been a strong demand for glass-fiber reinforced PBT with high and constant transparency to the NIR laser light. Constant laser transparency across a range of thicknesses and processing conditions is essential for consistent weld strengths.
One approach aiming at increasing the laser transparency is based on refractive indices matching of the amorphous polymer and crystalline PBT, and also of the fillers. For instance, JP2005/133087 discloses a resin composition incorporating PBT, PC, elastomer and high-refractive index silicone oil. However, the increase in transmission in the NIR region achieved results in the loss of mechanical properties.
Alternatively, speeding up the crystallization rate of PBT by employing a nucleating agent, results in improved laser transparency. Examples of such compositions are found in U.S. Pat. Nos. 8,889,768, 8,791,179, and 8,318,843. This is achieved by a chemical reaction between the nucleating agent and polymeric end groups of PBT to produce ionic end groups, which increases the rate of crystallization. Nevertheless, such chemical nucleation agents have disadvantages because they can significantly degrade many of the amorphous materials used in PBT blends such as polycarbonates and polyester carbonates. The outcomes of such degradations are unstable melt viscosities and deformations due to turbulent flow (splay and jetting). Another disadvantage of very quick crystallization induced by nucleating agents is the tendency to freeze stresses into the part, which can result in warpage.
Another approach to increase transparency of PBT is based on blending PBT with an amorphous polyester polycarbonate. Such compositions are disclosed in U.S. Patent Publ. No. 2011/0256406, DE 10230722 (U.S. Patent. No. 20070129475), U.S. Patent No. 20050165176 and U.S. Pat. No. 7,396,428.
In addition, it has been disclosed in US Patent Publication No. 2014-0179855 that near-infrared transparency (800-2500 nm) of the glass filled polymer blend containing crystalline or semi-crystalline polymer and amorphous polymers is significantly improved upon replacement of the amorphous phase having a total Fries content of less than 100 ppm by an amorphous phase with a total Fries content of 5400 ppm, as produced by melt polymerization. Nevertheless, the main problem of such compositions is the inconsistent transparency in the area where laser light operates.
In view of the above and the challenges involved, there has been a strong demand to achieve improved NIR transmission for laser weldable blends, particularly glass filled polycarbonate/poly(butylene terephthalate) (PC/PBT) blends, that offer heat resistance. Even though there are examples of blends that display the specified characteristics with respect to laser welding, these solutions still leave something to be desired—not only because of the intrinsically low laser transparency of the material, but also because of the difficult-to-control consistency of this property upon manufacturing/production of the materials.